This invention relates to light-emitting semiconductor devices, or light-emitting diodes (LEDs) according to more common parlance, and more particularly to such devices of the class suitable for use in displays and lamps, among other applications. The invention also concerns a method of making such light-emitting semiconductor devices.
The LED has been known which has a light-generating semiconductor region comprised of an active layer sandwiched between an n-type cladding or lower confining layer, which overlies the substrate, and a p-type cladding or upper confining layer. Part of the light generated in the active layer mostly directly traverses the upper confining layer and issues from one of the opposite major surfaces of the semiconductor region. The rest of the light is radiated more or less toward the substrate via the lower confining layer. How to redirect the highest possible percentage of this light component back toward the light-emitting surface of the semiconductor region is of critical importance for maximizing the efficiency of the LED.
One conventional approach to this goal was to place a reflector layer on the underside of the substrate, for reflecting the light that has traveled through the substrate. An obvious drawback to this approach was the inevitable light absorption by the substrate, both before and after reflection by the reflector. Lying in the way of bidirectional light travel, the substrate considerably lessened the efficiency of the LED. Another difficulty was the high electrical resistance between the light-generating semiconductor region and the substrate. This second mentioned difficulty brought about an additional inconvenience in those LEDs in which the substrate provides part of the current path, with a cathode, say, attached to the substrate. The forward voltage between the anode and cathode became inconveniently high in such LEDs.
Japanese Unexamined Patent Publication No. 2002-217450, filed by the assignee of the instant application, represents an improvement over the more conventional solution described above. It teaches the creation of a sparse or open-worked layer of gold-germanium-gallium alloy on the underside of the light-generating semiconductor region. This sparse alloy layer, as well as the surface parts of the semiconductor region left uncovered thereby, is covered by a solid reflector layer of aluminum or other metal. An electroconductive silicon baseplate is bonded to the underside of the solid metal-made reflector layer.
Making good ohmic contact with the light-generating semiconductor region of Groups III-V compound semiconductors such as, say, aluminum gallium indium phosphide, the open-worked gold-germanium-gallium alloy regions serve to make the LED lower in forward voltage requirement. The reflector layer itself of this prior art LED reflects the light impinging thereon from the light-generating semiconductor region via the open-worked alloy layer, instead of via the substrate as in the more conventional device. A significant improvement was thus gained in the efficiency of conversion from electric to optical energy.
The last cited prior art LED proved to possess its own weaknesses, however. Although capable of low-resistance contact with the light-generating semiconductor region, the open-worked layer of gold-germanium-gallium alloy poorly lacks in reflectivity. The solid metal-made reflector layer on the other hand is reflective enough but incapable of low-resistance contact with the light-generating semiconductor region. These inconveniences combined to make it difficult for the LED to accomplish the desired dual objective of low forward voltage requirement and high efficiency light production.
What is worse, in the course of the manufacturing process of the prior art LED, which involves thermal treatments, the solid metal-made reflector layer and open-worked alloy layer were prone to interaction with the light-generating semiconductor region to the impairment of reflectivity at their interfaces. The manufacture of high-efficiency LEDs of this type was therefore no easy task.
Japanese Unexamined Patent Publication No. 2004-179365 teaches the creation of an ohmic electrode of platinum, rhodium or silver on the non-light-emitting surface of a nitride semiconductor region. Japanese Unexamined Patent Publication No. 2004-006919 also suggests the creation of an ohmic electrode of gold, platinum, silver or nickel between the nitride semiconductor region and a substrate of gallium arsenide or the like. The ohmic electrode of such placement is required to be favorable not only in contact ohmicity but in reflectivity too. Contrary to this dual requirement, silver in particular is susceptible to oxidation or sulfurization. The silver electrode on oxidation or sulfurization will deteriorate in both reflectivity and contact ohmicity and also make it difficult to bond the nitride semiconductor region to the substrate.